Reactance filter having an improved edge steepness

ABSTRACT

A reactance filter, which is constructed from BAW resonators, has at least one basic element that has a first resonator in a first branch and a second resonator in a second branch. In one branch, there is situated a resonator having a greater ratio of dynamic to static capacitance than in the second branch, so that a filter is obtained having a resulting passband in which one edge is set steeper than the other edge. The selection of the edge that is to be set steeply takes place through the allocation of the first branch to the serial or to the parallel branch.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to bulk acoustic wave filters (alsoknown as BAW filters) that are constructed according to the reactancefilter principle.

[0002] From an article by K. M. Lakin et al. in Microwave SymposiumDigest, IEEE MTT-S International 1995, pp. 883-886, it is known toconstruct reactance filters from BAW resonators. Here, these resonatorsare used as impedance elements, and are for example wired or connectedto form ladder-type or lattice filters. This type of wiring for themanufacture of filters is also known as branching technology.

[0003] According to FIG. 1a, in its simplest specific embodiment a BAWresonator R is made up of a thin film P of a piezoelectric material,which is provided with an electrode E1, E2 on its upper and lower siderespectively. Ideally, this structure is surrounded by air on bothelectrode sides. When an electrical voltage is applied to theelectrodes, an electrical field acts on the piezoelectric layer, withthe result that the piezoelectric material converts a part of theelectrical energy into mechanical energy in the form of acoustic waves.These waves propagate parallel to the field direction, as what are knownas bulk waves, and are reflected at the electrode/air boundary surfaces.At a particular frequency f_(r), which is dependent on the thickness ofthe piezoelectric layer or on the thickness of the bulk resonator, theresonator exhibits a resonance, and thus behaves like an electricalresonator.

[0004] In the equivalent circuit diagram according to FIG. 1b, the BAWresonator R is made up of a series resonance circuit of dynamicinductance L1, dynamic capacitance C1, and dynamic resistance R1, aswell as a static capacitance C0, connected thereto in parallel. Theseries resonance circuit reproduces the behavior of the resonator in theresonance case, i.e., in the range of resonance frequency f_(r). Staticcapacitance C0 reproduces the behavior in the range f<<f_(r) andf_(r)>>f. Dynamic capacitance C1 is thereby proportional to the staticcapacitance C0 of the BAW resonator.

C1˜C0.   (1.1)

[0005] For the resonance frequency f_(r) and the anti-resonancefrequency f_(a) of a BAW resonator, the following hold: $\begin{matrix}{f_{r} = {\frac{1}{2\pi \sqrt{{L1} \cdot {C1}}}\quad {and}}} & (1.2) \\{f_{a} = {f_{r}{\sqrt{1 + \frac{C1}{C0}}.}}} & (1.3)\end{matrix}$

[0006] According to FIG. 7, a reactance filter is made up of at leastone basic element that has a serially connected resonator R2 having aresonance frequency f_(rs) and an associated anti-resonance frequencyf_(as), and that has a second resonator R1 that is connected parallel toa second terminal, in particular parallel to ground, having a resonancefrequency f_(rp) and an associated anti-resonance frequency f_(ap). Inorder to produce a filter having a bandpass characteristic and a centerfrequency f₀, the following relation holds for the two resonators in theserial or in the parallel branch:

f_(ap)≈f_(rs)≈f₀   (1.4).

[0007]FIG. 16a shows the curve of the impedance Zs of the serialresonator and of the admittance Yp of the parallel resonator, plottedover the frequency f. FIG. 16b shows the passband response of a filtermade up of a basic element, whose resonance frequencies are selected asin FIG. 16a. FIG. 7 shows a basic element that is to be regarded inprinciple as a two-port network having terminals 3-1 or 3-2 as a port 1and having terminals 3-3 or 3-4 as a port 2. At the same time, terminal3-1 is the input and terminal 3-3 is the output of the series resonator.The input of the parallel resonator is connected with terminal 3-1.Terminals 3-2 and 3-4 represent the reference ground, given asymmetricaloperation. The output 3-5 of parallel resonator R1, which faces thereference ground, is designated in the following as the output or groundside of the parallel resonator. The inductance L_(ser), which issituated between the output side of the parallel resonator and thereference ground, reflects the connection to the housing ground in thereal construction.

[0008] The selection level of a reactance filter constructed from BAWresonators is determined on the one hand by the ratio C0 _(p)/C0 _(s) ofthe static capacitance C0 _(p) in the parallel branch and the staticcapacitance C0 _(s) in the series branch, and on the other hand by thenumber of basic elements that are cascaded, i.e., connected in serieswith one another.

[0009] A plurality of basic elements can be wired together in matchedfashion, whereby the structure of each of the second adjacent basicelements is mirrored. The output impedance of the first basic element(7-1 in FIG. 2, or 8-1 in FIG. 3) is then equal to the input impedanceof the second basic element (7-2 in FIG. 2 or 8-2 in FIG. 3), so thatonly minimal losses are produced by mismatching. Many structures areknown for the wiring of a plurality of basic elements. Some examples areshown in FIGS. 4 and 5.

[0010] Resonators of the same type (series resonators or parallelresonators) that are situated immediately one after the other in acircuit of a reactance filter can also be respectively combined to forma resonator, whereby the overall capacitive effect of the combinedresonator remains constant.

[0011] From equations (1.2) to (1.4), it can be seen that both themaximum achievable bandwidth and also the steepness of the edges of sucha reactance filter depend on the difference of the resonance andanti-resonance frequencies of the individual resonators. This differencein turn results from the ratio of dynamic capacitance C1 and staticcapacitance C0. Because these capacitances are proportional to oneanother, the capacitance ratio C1/C0 does not change when one of thesecapacitances is altered. For example, C0 could be varied by changing thesize of the resonator. As a rule, all resonators of a reactance filterhave the same relative bandwidth

(fa−fr)/f0.

[0012] Curve 1 in FIG. 6 shows the passband response of a reactancefilter that is constructed from uniform BAW resonators, with eachresonator having a relatively large ratio of dynamic to staticcapacitance. The individual resonators thus have a relatively largebandwidth. Curve 2 is the passband curve of a corresponding reactancefilter made up of resonators having a small ratio of dynamic to staticcapacitance, and thus a relatively low bandwidth of the individualresonators. In the first case (curve 1), a bandpass filter is obtainedhaving a high bandwidth and a low edge steepness, while in the secondcase (curve 2) a bandpass filter is obtained having a low bandwidth anda high edge steepness.

[0013] If it is now attempted, in such a steep-edged filter, to increasethe bandwidth to the level of the filter having the larger capacitanceratio by increasing the center frequencies of series resonators and/orreducing the center frequency of the parallel resonators, a strongmismatching results in the center of the passband, because nowf_(ap)<<f_(rs). Condition (1.4) is thus no longer fulfilled. For thisreason, the losses in the center of the passband also increase morestrongly.

[0014] Another possibility for broadening a steep-edged filter consistsin a reduction of the ratio (C0 _(p)/C0 _(s)) of the static capacitanceC0p in the parallel branch and the static capacitance C0 _(s) in theseries branch. In this way, the bandwidth can be enlarged to a certainextent without losing the self-matching and the small losses connectedtherewith. However, with this measure the selection level of the BAWreactance filter is strongly reduced, so that the filter can no longermeet possible selection demands, and can for example no longersufficiently attenuate undesired frequencies.

SUMMARY OF THE INVENTION

[0015] The object of the present invention is therefore to provide areactance filter constructed from BAW resonators that has an improvededge steepness with sufficient bandwidth, without having to accept anadditional matching or a reduction of the selection level for thispurpose.

[0016] This object is achieved according to the present invention by areactance filter constructed from resonators of the BAW type, the filtercomprises at least one basic element having a first resonator in a firstbranch and a second resonator in a second branch, one of the branchesbeing a serial branch and the other branch being a parallel branch, eachresonator having a specific ratio V_(C)=C1/C0 of a dynamic to staticcapacitance, the ratio V_(C) for the resonator of the second branch isset smaller than the ratio for the first branch.

[0017] The present invention exploits the fact that for the RF filtersin many mobile radiotelephone systems, high demands are placed only onthe band demarcation from the corresponding other duplex band. That is,as a rule an RF filter requires a steep edge only on the side of thepassband facing the other duplex band. In the currently standard mobileradiotelephone systems based on GSM, CDMA, AMPS, or TDMA, in the case ofa receive filter this is the left edge, while in the case of a transmitfilter it is the right edge.

[0018] The present invention makes use of this fact, and indicates areactance filter that is constructed from resonators of the BAW type. Itcomprises at least one basic element having a first resonator in a firstbranch and having a second resonator in a second branch that areconnected parallel to one another, one of the branches being the serialbranch while the other branch is a parallel branch. Each of theresonators has a specific ratio V_(C) of dynamic capacitance C1 tostatic capacitance C0:

V _(C) =C 1/C 0

[0019] whereby according to the present invention the ratio V_(C) forthe resonator of a first branch is set lower than for the resonator ofthe second branch. Dependent on the branch in which the ratio V_(C) hasbeen set lower, the reactance filter according to the present inventionhas a passband response having an improved edge steepness for one edge.The other edge, as well as the remaining resonator and filtercharacteristics, remain unaffected by this change. If, for example, in aresonator in the serial branch the ratio V_(C) is reduced in relation tothe corresponding ratio V_(C) in the resonator of the parallel branch,the right edge of the passband is set steeper, i.e., the edge thatdemarcates the passband from higher frequencies. Analogously, in areactance filter in which the resonator in the parallel branch has asmaller ratio V_(C) than does the resonator in the serial branch, apassband is obtained having a left edge that is set steeper. Because, ina basic element of a reactance filter, the resonance and anti-resonancefrequencies of the parallel resonator are lower than the correspondingfrequencies of the serial resonator, for example the right edge of thepassband is determined by the characteristics of the serial resonator.The steepness of the right edge can be seen in the speed with which theimpedance curve of the serial resonator climbs from the resonancefrequency to the anti-resonance frequency. A steeper impedance increasein a (serial) resonator is obtained when the distance between theresonance and the anti-resonance frequency of the resonator is reduced.Because, conversely, the steepness of the left edge is determinedessentially by the parallel resonator or by the resonator in theparallel branch, a steeper setting of the left edge is achieved througha reduction of the distance between the resonance and anti-resonancefrequency of the parallel resonator.

[0020] Because as a rule a real reactance filter is obtained by wiring aplurality of basic elements together, a reactance filter standardlycomprises a plurality of serial resonators and a plurality of parallelresonators. A reactance filter according to the present invention isthen already obtained when the cited modifications have been carried outin a single resonator of one type (serial or parallel). A furtherimproved, even steeper edge is obtained if a plurality of resonators ofone type, preferably all resonators of one type, have a smaller distancebetween the resonance and anti-resonance frequency. Through the givendependence of the corresponding quantities on one another, this distanceincreases with the cited ratio V_(C) of the dynamic to the staticcapacitance. A resonator having such a reduced distance is designated anarrowband resonator. A resonator having a correspondingly largerdistance of the resonance and anti-resonance frequency is designated abroadband resonator.

[0021] Independent of the use of at least one narrowband resonator for afirst branch, the broadbandedness of the filter, i.e., the width of thepassband, is achieved in that broadband resonators are used in thesecond branch.

[0022] With a filter according to the present invention, having forexample an improved, steeper right edge, a higher selection is achievedat frequencies that are somewhat higher than the highest frequency ofthe passband. This is for example advantageous in a filter that is usedin current GSM-based or CDMA-based mobile radiotelephone systems as thefilter in the transmission path, which must provide a high degree ofsuppression of the receive band.

[0023] Conversely, a filter according to the present invention, havingfor example an improved, steeper left edge, is achieved throughnarrowband parallel resonators resulting in a high selection atfrequencies that are somewhat lower than the lowest frequency of thepassband of the filter. Such filters are preferably used as filters inthe receive path of current GSM-based or CDMA-based mobileradiotelephone systems, which must provide a high degree of suppressionof the transmit band.

[0024] Via the known connection, according to the following equation, ofthe effective coupling coefficient K² _(eff) with the position of theresonance and anti-resonance frequency: $\begin{matrix}{K_{eff}^{2} = {\left( {\pi/2} \right)^{2} \times {\frac{{fa} - {fr}}{fa}.}}} & (1.5)\end{matrix}$

[0025] It results that a narrowband resonator can also be achievedthrough the direct influencing of the effective coupling coefficient K²_(eff). A narrowband resonator can be realized on a suitablepiezoelectric material having a lower effective electromechanicalcoupling coefficient. This effective electromechanical couplingcoefficient is in turn obtained from the sum of the effective couplingsof all modes capable of propagation in a piezoelectric material.

[0026] Because as a rule a real filter uses only one mode, while incontrast the center frequencies of the remaining modes are at asufficient distance from the passband, the effective coupling (for themode used) can be determined from the equivalent circuit diagram of aBAW resonator according to the following equation: $\begin{matrix}{k_{eff}^{2} \approx {\frac{C1}{C_{1} + C_{0}}.}} & (1.6)\end{matrix}$

[0027] From this formula there results the dependence of thenarrowbandedness of a resonator on the ratio of the dynamic to staticcapacitance of a resonator, or, precisely stated, on the ratio ofdynamic to static capacitance of the relevant oscillation mode, or theoscillation mode that is in use, of the resonator. From thisconsideration it results that a BAW resonator having a smaller ratioV_(C) has a lower effective coupling k² _(eff). If for the constructionof a resonator a piezomaterial is used having a small couplingcoefficient, and thus low effective coupling, a resonator is obtainedhaving a small distance between the resonance frequency and theanti-resonance frequency. Given the use of a higher-couplingpiezomaterial, a resonator is obtained having a greater distance betweenthe resonance frequency and the anti-resonance frequency.

[0028] A reactance filter according to the present invention thereforehas, for example, resonators having a higher-coupling piezomaterial inthe series branch of the reactance filter, and, in contrast, resonatorshaving lower-coupling piezomaterial in the parallel branch of the samefilter. Such a filter then has a high steepness of the left edge. At thesame time, the reactance filter according to the present invention has ahigh bandwidth, ensured by the relatively great distance between theresonance frequencies and the anti-resonance frequency in the seriesresonators.

[0029] Moreover, in a reactance filter having BAW resonators theeffective coupling can be reduced, when an additional layer which ismade of a non-piezoelectric material is inserted between the twoelectrodes of a BAW resonator.

[0030] Here the coupling coefficient is reduced by the ratio of thelayer thickness of the non-piezoelectric material to the overall layerthickness of the resonator. In any case, with such a layer, a reductionof the effective coupling is obtained that for the filter or theresonator is equivalent to a reduction of the ratio V_(C), and thus isalso equivalent to a reduction of the distance between the resonance andthe anti-resonance frequency.

[0031] Another possibility for affecting the effective coupling consistsin the selection of suitable electrode materials for the BAW resonators.A high electromechanical coupling is achieved using an electrodematerial that effects a high mechanical impedance of the electrode forthe mode used. An electrode material that increases the effectivecoupling (for the relevant or employed mode in the resonator) isobtained dependent on the position of the electrode metal in theperiodic table of the elements, or is determined as an empirical value.A reactance filter according to the present invention therefore has forexample resonators that in a first branch use an electrode material thatdiffers from the electrode material of the resonators in the secondbranch. For example, through the use of a heavy electrode material, suchas for example tungsten, the effective coupling is increased, whereby aresonator is obtained that is more broadbanded in comparison with aresonator having aluminum electrodes. A reactance filter havingresonators having tungsten electrodes in a first branch and havingresonators having aluminum electrodes in a second branch accordingly hasa more narrowband resonator in the second branch. If the second branchis a parallel branch, the left edge of the passband of the reactancefilter is improved. If the correspondingly narrowbanded resonator isused in the serial branch, the right edge is improved in the reactancefilter.

[0032] A BAW filter is preferably surrounded by air on both sides of theelectrodes. For this purpose, in the technical realization two supportpoints situated at a large distance from one another are provided for anelectrode layer; here one speaks of what are called bridge resonators.In these bridge resonators, the acoustic wave is reflected on both sidesof the resonator at the solid element/air transition. However, it isalso possible to construct a BAW resonator in such a way that one of theelectrodes is situated with its entire surface on a substrate. Thereflection of the acoustic wave can then be ensured using an acousticmirror, which can for example be realized by two layers having differentacoustic impedance, with each layer having a layer thickness of λ/4 inrelation to the wavelength λ of the acoustic wave inside the layermaterial. The repeated reflections at the transitions of the two layershaving sharply differing acoustic impedances then results in theextinguishing of wave portions reflected at different boundary surfaces,which in turn means a high degree of reflection for the mirror.

[0033] However, in the use of a resonator having an acoustic mirror apart of the mechanical energy of the resonator is located outside theelectrodes. Within the layer sequence electrode/piezomaterial/electrode,the ratio of the electrical to the mechanical energy therefore changes,and thus the effective coupling, measured according to the followingequation, also changes: $\begin{matrix}{k_{eff}^{2} \approx {\frac{u_{E}}{u_{E} + u_{M}}.}} & (1.7)\end{matrix}$

[0034] Here u_(E) denotes the electrical energy density, and u_(M)denotes the mechanical energy density. From this equation, it clearlyfollows that the effective coupling of a resonator having an acousticmirror is reduced in relation to a bridge resonator by the amount u_(M).This means that resonators having an acoustic mirror have a lowereffective coupling, and thus a smaller distance between the resonanceand anti-resonance frequency, than do bridge resonators. A reactancefilter according to the present invention therefore has bridgeresonators for example in the parallel branch, and in contrast has inthe serial branch resonators having an acoustic mirror, whereby abandpass response is obtained having a steeper passband in the rightedge.

[0035] It is also possible to influence the effective coupling k² _(eff)of a resonator through the use of different acoustic mirrors. This cantake place through the use of mirror layers having differentthicknesses, or through the use of mirror layers having differentmaterial. A reactance filter according to the present invention is thendistinguished by resonators using, in a first branch, acoustic mirrorsthat are at least partly different than those used in a second branch.

[0036] In the following, the present invention is explained in moredetail on the basis of exemplary embodiments and the associated Figures.

BRIEF DESCRIPTION OF THE DRAWINGS

[0037]FIGS. 1a to 1 c show a BAW resonator with FIG. 1a being aschematic cross-sectional view, FIG. 1b being an equivalent circuitdiagram thereof, and FIG. 1c illustrating a substitute symbol used for aresonator.

[0038]FIGS. 2 and 3 show circuit diagrams for two possibilities for thewiring of two basic elements to form a filter.

[0039]FIG. 4 shows a circuit diagram for a reactance filter having threebasic elements.

[0040]FIG. 5 shows a circuit diagram for a reactance filter having fourbasic elements.

[0041]FIG. 6 is a graph showing the attenuation curves for a broadbandfilter and for a narrowband filter.

[0042]FIG. 7 shows a circuit diagram for a basic element of a reactancefilter, constructed from BAW resonators.

[0043]FIG. 8 shows a circuit diagram for a simplified filter structurehaving three basic elements.

[0044]FIG. 9 shows a circuit diagram for the same filter having asimplified structure.

[0045]FIGS. 10 and 11 are graphs showing the bandpass responses ofreactance filters according to the present invention.

[0046]FIG. 12 is a graph showing the impedance curves of resonatorshaving different electrode materials.

[0047]FIG. 13 is a cross-sectional view of a resonator having anadditional dielectric layer.

[0048]FIG. 14 is a cross-sectional view of a bridge resonator inschematic cross-section.

[0049]FIG. 15 is a cross-sectional view of a BAW resonator having anacoustic mirror.

[0050]FIGS. 16a and 16 b are graphs with FIG. 16a showing superimposedadmittance and impedance curves for individual resonators, and FIG. 16bshowing the attenuation characteristic of a reactance filter.

DESCRIPTION OF THE PREFERRED EMBODIMENTS First Exemplary Embodiment

[0051] Resonators constructed as in FIG. 1a are wired together to form areactance filter (see FIG. 1b). Each resonator comprises a firstelectrode layer E1, a piezoelectric layer P and a second electrode layerE2 (FIG. 1a). In FIG. 1c, the symbol standardly used for resonators isshown.

[0052]FIG. 7 shows a basic element constructed from a first resonator R1in a parallel branch and a second resonator R2 in a serial branch.Terminals 3-1 and 3-2 form the input of the filter, and terminals 3-3and 3-4 form the output of the filter. The parallel branch, or resonatorR1 in the parallel branch, is connected with terminals 3-2 or 3-4 via aseries inductance L_(ser), formed from the sum of the inductances of theconnection to the housing ground. According to the present invention, inthis exemplary embodiment resonator R1 is formed with a piezoelectriclayer P made of zinc oxide, which has an electromechanical couplingconstant K² _(eff) 1, and resonator R2 is formed with a piezoelectriclayer P made of aluminum nitride, which has a piezoelectric couplingconstant K² _(eff) 2, such that K² _(eff) 1>K² _(eff) 2. Via therespective thickness of the piezoelectric layer, or the thickness of theoverall resonator, the resonance frequencies, and thus also theanti-resonance frequencies, of the two resonators R1 and R2 are set suchthat the resonance frequency of R2 is approximately equal to theanti-resonance frequency of R1.

[0053] Passband curve 2 in FIG. 10 shows the attenuation characteristicof a reactance filter according to this exemplary embodiment of thepresent invention, presented in comparison with passband curve 1 of aconventional reactance filter, in which both resonators use zinc oxidefor the piezoelectric layer P of the resonators. It can be seen that theright edge of curve 2 is set significantly steeper than that of theknown filter. The bandwidth of the overall filter is reduced onlyinsignificantly.

Second Exemplary Embodiment

[0054] Again, a reactance filter is constructed having a basic elementwired according to FIG. 2, and both resonators are fashioned as inFIG. 1. In contrast to the first exemplary embodiment, both resonatorsdo comprise the same piezoelectric material for the layer P, but differin the electrode material used for electrodes E1 and E2. While aluminumis used for resonators R1, tungsten is used as the electrode materialfor resonators R2. Because k² _(eff) 2>k² _(eff) 1 holds for theeffective coupling k² _(eff), as a result a reactance filter is obtainedwhose passband curve 2 is shown in FIG. 11. It can be seen that curve 2of the filter according to the present invention has a left edge that isset significantly steeper than the left edge of curve 1, which shows thepassband response of a known reactance filter in which the sameelectrode material (tungsten) was used for both resonators.

[0055]FIG. 12 shows the influence of the electrode material on theimpedance characteristic of a resonator. Curves 3 and 4 show theimpedance characteristic of resonators fashioned according to FIG. 1,whereby curve 3 shows the impedance of a resonator having aluminumelectrodes while curve 4 shows the impedance characteristic of aresonator having tungsten electrodes. It can be seen that the greatereffective coupling of tungsten electrodes according to curve 4 resultsin a greater distance of the resonance frequency from the anti-resonancefrequency.

Third Exemplary Embodiment

[0056] A resonator is formed according to FIG. 13. This resonatorcomprises, between a first electrode E1 and a second electrode E2, madefor example of aluminum, a piezoelectric layer P made for example ofaluminum nitride, as well as a dielectric layer D, made for example ofsilicon oxide. If the layer portion of the silicon oxide layer is 16%,the coupling coefficient k² _(eff) decreases from a value of 0.0645,determined in a resonator according to FIG. 1 having aluminum nitride asa piezoelectric layer, to a value of 0.057 for the resonator accordingto the present invention, as shown in FIG. 13. This latter resonatortherefore has a smaller distance between the resonance and theanti-resonance frequency, and can be used in combination withconventional resonators (see FIG. 1), whereby in the reactance filter(for example according to FIG. 7) the serial and parallel resonators areformed differently, i.e., respectively according to FIG. 1 or FIG. 13.

Fourth Exemplary Embodiment

[0057]FIG. 14 shows a BAW resonator formed as a bridge resonator. Thisresonator has a basic element corresponding to FIG. 1, but is howeverconnected with a substrate S via two socket structures F. Because thepredominant part of the lower electrode E1 of the resonator has air as aboundary surface, this bridge resonator behaves approximately as aresonator that can oscillate completely freely. At the two boundarysurfaces E1/air or E2/air, total reflection of the acoustic wave therebytakes place.

[0058]FIG. 15 shows a resonator that is situated on a substrate S withthe aid of an acoustic mirror AS.

[0059] A reactance filter according to the present invention is nowmanufactured from at least one basic element (for example according toFIG. 7), whereby in a first branch, resonators having a bridgeconstruction are used, while, in contrast, in a second branch,resonators having an acoustic mirror are used. Because the effectivecoupling coefficient for resonators according to FIG. 14 is greater thanfor resonators according to FIG. 15, the passband edge allocated to thebranch having the resonators with the acoustic mirror can be formed moresteeply. If for example resonators R1 are realized with an acousticmirror, and resonators R2 are constructed as bridge resonators, asteeper left edge is obtained in the passband response of the reactancefilter constructed in this way.

[0060] Although the present invention has been presented and explainedonly on the basis of a few exemplary embodiments, it is of course notlimited to these. Possible constructions of the present invention relateto additional methods for varying the bandwidth of an individualresonator, and correspondingly to the use of resonators having differentbandwidths in inventive filters. The variations can thereby compriseindividual resonators in one branch, individual resonators in bothbranches, all resonators in one branch, or all resonators in bothbranches.

I claim: 1-14. (cancelled).
 15. A reactance filter comprising at leastone basic element having a first resonator in a first branch and asecond resonator in a second branch, one of the branches being a serialbranch and the other branch being a parallel branch, each resonatorbeing of a BAW type having a specific ratio V_(C)=C1/C0 of dynamic tostatic capacitance and the ratio V_(C) for the resonator of the secondbranch being set smaller than the ratio V_(C) for the resonator of thefirst branch.
 16. A reactance filter according to claim 15, wherein theresonator of the first branch is made of a first piezoelectric materialand the resonator of the second branch is made of a second piezoelectricmaterial differing therefrom, each piezoelectric material having acoupling coefficient with the coupling coefficient of the firstpiezoelectric material being higher than the coupling coefficient of thesecond piezoelectric material.
 17. A reactance filter according to claim15, wherein the electrode materials for the resonators of the first andsecond branch are different, with the electrode material for theresonators of the first branch producing a higher effective couplingthan the electrode material of the resonators of the second branch. 18.A reactance filter according to claim 15, in which the BAW resonators ofthe second branch include, besides a layer of a piezoelectric material,another layer of an additional material that has a lower dielectricconstant than the piezoelectric material between the two electrodes. 19.A reactance filter according to claim 15, wherein at least theresonators of the second branch include an acoustic mirror underneath anelectrode layer and wherein the effective coupling coefficient for theresonators of the second branch is lowered in relation to the couplingcoefficient of the resonators of the first branch.
 20. A reactancefilter according to claim 19, wherein the resonators of the first andsecond branch have acoustic mirrors that are different with respect toone of the layer thicknesses of the mirror layers, the reflectioncharacteristics in the two branches and the layer thickness of themirror layer and reflection characteristics.
 21. A reactance filteraccording to claim 19, wherein only the resonators of the second branchhave an acoustic mirror, and the resonators of the first branch have adifferent method for the reflection of acoustic waves.
 22. A reactancefilter according to claim 15, wherein each branch has a plurality ofbasic elements wired with one another, with the basic elements of theserial branch being connected to one another in series, and the elementsof the parallel branch being connected in parallel.
 23. A reactancefilter according to claim 22, wherein the ratio V_(C)=C1/C0 of dynamicto static capacitance in at least one resonator of the serial branch isset to a different value than the corresponding ratio of the resonatorsin the parallel branches.
 24. A reactance filter according to claim 15,wherein to obtain a passband response having a passband with a steeperleft edge, the ratio V_(C)=C1/C0 of dynamic to static capacitance islowered in at least one resonator of the parallel branches in relationto the ratio for the resonators of the serial branch.
 25. A reactancefilter according to claim 15, wherein to obtain a passband responsehaving a passband with a steeper right edge, the ratio V_(C)=C1/C0 ofdynamic to static capacitance in at least one resonator of the serialbranch is lowered in relation to the ratio for the resonators of theparallel branches.
 26. A reactance filter according to claim 15, whereinthe resonators of the parallel branches are connected in series with aninductance, and are respectively connected individually with a groundterminal.
 27. A wireless communication system having a transmit part anda receiver part, each part having a reactance filter comprising at leastone basic element having a first resonator in a first branch and asecond resonator in a second branch, with one of the branches being aserial branch and the other branch being a parallel branch, eachresonator being a BAW type having a specific ratio V_(C)=C1/C0 of adynamic to static capacitance and the ratio V_(C) for the resonator ofthe second branch being set smaller than the ratio of the ratio V_(C)for the resonator of the first branch, the filter for the transmit parthaving the second branch being a serial branch and the first branchbeing the parallel branch, so that the filter has a steeper right edge,and the filter for the receive part having the second branch being theparallel branch and the first branch being the serial branch, so thatthe filter has a steeper left edge.
 28. A duplexer having two passbandfilters, each passband filter being a reactance filter comprising atleast one basic element having a first resonator in a first branch and asecond resonator in a second branch, one of the branches being a serialbranch and the other branch being a parallel branch, each resonatorbeing of a BAW type having a specific ratio V_(C)=C1/C0 of a dynamic tostatic capacitance and the ratio V_(C) for the resonator of the secondbranch being smaller than the ratio V_(C) for the resonator of the firstbranch, one of the passband filters having a low center frequency andbeing a reactance filter, with the second branch being a serial branchand the first branch being a parallel branch, so that it has a steeperright edge and the other of the two passband filters having a highcenter frequency and being a reactive filter with the second branchbeing a parallel branch and the first branch being a serial branch, sothat the other filter has a steeper left edge.